1. Field of the Invention
The present invention relates to a modem apparatus, communication control apparatus and communication terminal apparatus with the function of performing communications based on the communication procedure conforming to a predetermined Recommendation of ITU-T Recommendations and the communication control method therefor.
2. Description of the Related Art
A communication terminal apparatus provided with a modem for performing communications based on the communication procedure conforming to the predetermined Recommendation of ITU-T Recommendations has conventionally performed communications based on the communication procedure conforming to the predetermined Recommendation as an answering terminal by exchanging procedure signals, which are expected in the communication procedure, with a calling terminal.
For example, as illustrated in FIG. 1A, the communication terminal apparatus provided with a modem for performing communications based on the communication procedure conforming to ITU-T Recommendation V.32 (hereinafter referred to as “Recommendation V.32”) as an answering terminal transmits an ANS signal to a calling terminal in response to a call placed from the calling terminal. The calling terminal transmits an AA signal to the answering terminal after detecting the ANS signal. The answering terminal transmits an AC signal to the calling terminal after detecting the AA signal. Further, as illustrated in FIG. 1B, the communication terminal apparatus provided with the modem for performing communications based on the communication procedure conforming to Recommendation V.32 as the answering terminal transmits the ANS signal to the calling terminal in response to the received signal, and when a predetermined time passes after transmitting the ANS signal, further transmits the AC signal to the calling terminal. The calling terminal transmits the AA signal to the answering terminal after detecting the AC signal. Thus, the calling and answering terminals mutually transmit and receive expected procedure signals, thereby performing communications based on the communication procedure conforming to Recommendation V.32.
However, in the conventional communication terminal apparatus, when the procedure signals are transmitted and received according to a sequence specified in the predetermined Recommendation, the case sometimes occurs that the procedure signal transmitted from the answering terminal is not correctly identified at the calling terminal. Thereby, the calling terminal cannot transmit the procedure signal expected by the answering terminal, resulting in the problem that the communication causes the looped condition thereof.
In the above-mentioned example, for example, in the case illustrated in FIG. 1C, when the answering terminal transmits the ANS signal to the calling terminal, the case sometimes occurs that the calling terminal does not recognize the ANS signal correctly, and transmits the CM signal specified in Recommendation V.8 to the answering terminal. In this case, the answering terminal continues to transmit the AC signal to the calling terminal because the AA signal expected from the AC signal is not transmitted from the calling terminal. Meanwhile, the calling terminal continues to transmit the CM signal to the answering terminal because a JM signal expected from the CM signal is not transmitted from the answering terminal. Thereby, the communication causes the looped condition thereof.
Such a problem occurs similarly in the communication terminal apparatus provided with a modem for performing communications based on the communication procedure conforming to Recommendation T.30. Namely, as illustrated in FIG. 2A, the answering terminal continues to transmit a DIS signal to the calling terminal after transmitting the DIS signal because a DCS signal expected from the DIS signal is not transmitted from the calling terminal. Meanwhile, the calling terminal also continues to transmit the CM signal to the answering terminal because the JM signal expected from the CM signal is not transmitted from the answering terminal. Further, also in the case illustrated in FIG. 2B, the answering terminal continues to transmit the DIS signal after transmitting the DIS signal because the DCS signal expected from the DIS signal is not transmitted from the calling terminal. Meanwhile, the calling terminal continues to transmit the AA signal because the AC signal expected from the AA signal is not transmitted from the answering terminal. Thus, the communication causes the looped condition thereof.